The conventional approach to detecting the state of optical excitation of targets, where the target is an atom, ion or centre in a solid, involves laser irradiation to promote an electron to a higher energy level. When the electron returns to its original energy level a single photon is emitted, and detection of that single photon is attempted. The efficiency of the detection process depends on several factors:                the quantum efficiency of the emission, that is the probability of the target emitting a detectable photon.        the collection efficiency, that is the probability of the detectable photon arriving at the detector, and        the detector efficiency, that is the probability of the detector detecting the photon.        
Since the overall probability is generally low it is necessary to repeat the measurement many times in order to determine whether the target is being excited by the light.
Single charge detection with electrical control is known using single-electron transistors.